1. Field of the Invention
The present invention relates an optical device and a method of controlling the optical device.
2. Description of the Related Art
A wavelength tunable semiconductor laser is one of optical devices. The wavelength tunable semiconductor laser has a gain for a laser oscillation and selects wavelength. There is a method of tuning wavelength characteristics of loss, reflection or gain by tuning refractive index of an optical functional region such as a diffractive grating provided in an optical waveguide in a resonator, as a method of selecting wavelength.
The method of tuning the refractive index does not need a mechanical movable portion, being different from a method of tuning a mechanical angle or a mechanical length. Therefore, the method has an advantage in reliability and a manufacturing cost. There is a method of tuning a temperature of an optical waveguide, a method of tuning a carrier density in an optical waveguide with current injection or the like, as a method of tuning refractive index of an optical waveguide. There is proposed a semiconductor laser having a Sampled Grating Distributed Bragg Reflector (SG-DBR) in which peak wavelength of reflection peak ranges periodically and a Sampled Grating Distributed Feedback (SG-DFB) in which peak wavelength of gain spectrum ranges periodically, as a concrete example of a wavelength tunable laser adopting a method of tuning a temperature of an optical waveguide.
This semiconductor laser controls a correlation between the reflection spectrums of the SG-DBR and the SG-DFB, selects a wavelength with a vernier effect, and emits a laser light. That is, the semiconductor laser oscillates at one of wavelengths where two spectrums are overlapped and reflection intensity gets biggest. It is therefore possible to control the oscillation wavelength by controlling the correlation of two reflection spectrums.
Japanese Patent Application Publication No. 9-92934 (hereinafter referred to as Document 1) discloses a semiconductor laser controlling an oscillation wavelength with a control of refractive index of an optical waveguide. In Document 1, a heater is adopted as a control portion of the refractive index of the optical waveguide. The wavelength is controlled with a control of a temperature control of the optical waveguide with use of the heater.
Degradation of the heater is a problem, in a case where the heater is used for a control of the refractive index of the optical waveguide. Heat value of the heater changes even if a constant current is provided to the heater, when a resistance of the heater changes because of the degradation of the heater. In particular, temperature differential between each optical waveguide is important and the unexpected changing of the heat value is fatal, in an optical device that has a combination of the optical waveguides having different wavelength property from each other such as a combination of the SG-DFB and the SG-DBR.
The width of temperature range (ΔT) of the heater for controlling the temperature of the optical waveguide is approximately 40 degrees. The temperature of the heater is relatively low. Therefore, the degradation of the heater has not been considered.
There is a method of controlling heat value of a heater according to a detection result of a temperature detection element such as a thermistor arranged around an optical waveguide. The detection result of the temperature detection element is ideal because the detection result includes a changing of the temperature caused by temperature changing of outer circumstance and the degradation of the heater.
However, it is necessary to arrange the temperature detection element around the heater in order to realize the method. In this case, it is relatively difficult to apply a temperature detection element to a micro device such as a laser chip.